[0001] Due to environmental issues and especially due to the release of carbon dioxide in
conventional steel making processes avoidance of carbon dioxide being released becomes
more and more interesting for the steel industry.
[0002] One focus lies on the production of so-called direct reduced iron (DRI). This direct
reduced iron is produced from the direct reduction of an iron ore by a reducing gas
or elemental carbon produced from natural gas or coal. The direct reduction of iron
is a solid-state process which works below the melting point of iron. One example
is a direct reduction of an iron ore in the presence of a reducing gas, the so-called
Syngas, which is a mixture of hydrogen and carbon monoxide. This takes place at temperatures
of 800 - 1200°C.
[0003] These direct production processes can be divided in gas-based and coal-based processes
wherein in both cases, as in conventional steel making, the aim is to remove the oxygen
contained in various forms as iron oxide.
[0004] Besides avoiding the release of carbon dioxides, the direct reduction process is
relatively energy efficient. For example, the DRI process requires significantly less
fuel than a traditional blast furnace. By using the direct reduction process, it is
possible to avoid a typical integrated steel plant. For the shipping, handling and
storage direct reduced iron can be brought into a briquetted form which is normally
done in a high temperature process called hot briquetting providing hot briquetted
iron (HBI). For the production of HBI iron ore is reduced with hydrogen or with natural
gas which is much more environmentally friendly than using coke. The reduction process
is carried out in a reduction tower. The tower is the heart of the plant. The direct
reduction process is quite complex and includes the steps of pelletizing iron ore
or using pelletized iron ore and filling them into the reactor. Converting natural
gas into a reducing gas and then injecting the reducing gas into the system and circulating
it in a closed system. The hot reducing gas is led through the iron ore from the bottom
to the top of the reduction furnace using a counter flow principle. The iron ore is
reduced, and sponge iron is produced. Afterwards, the sponge iron is pressed into
briquettes in a high temperature process resulting in hot briquetted iron (HBI).
[0005] To achieve a proper density and the desired physical properties of the HBI the temperature
of the feed material to the briquetting press is important. The temperature of the
material arriving at the press is determined by the heat in the furnace above it,
which is primarily influenced by the gas flow and temperature required for the reduction.
The amount of natural gas injected below the reduction zone cools the material and
adds carbon. To increase the temperature of the bed, oxygen can be injected but its
use is limited by the temperature at which clustering of the material occurs. Further,
carbon monoxide combined with natural gas injection into the bottom of the furnace
can be used to limit the temperature drop when carbon is added but requires a complex
flow sheet including hydrogen separation via methods such as pressure swing absorption
(PSA), vacuum swing absorption (VSA) or other methods known in the art.
[0006] For example, the reactor zone temperature in the reduction furnace is about 850°C
wherein the HDRI temperature after natural gas injection and cooling is about 650
- 680°C. After natural gas injection and cooling a number of feed channels distribute
the material to the briquetting machines.
[0007] The prior art deals with various problems, for example, temperature imbalances which
may result in each briquetting machine receiving material at different temperatures.
The quality of the pellets influences briquetting and the thus the final HBI quality.
Further, the maximum temperature of the feed material is linked to the furnace operation.
[0008] It is an object of the invention to produce hot briquetted iron with more uniform
properties and to provide for a process which allows for better control of the temperature
of direct produced iron. It is a further object to allow for uniform material properties
before feeding into the hot briquetting process.
[0009] The objects are achieved by means of the process claimed in claim 1.
[0010] Advantageous features are claimed in the dependent claims.
[0011] The inventors found that due to flow issues within the discharge system temperature
imbalances arise having an impact on briquetting. Some of the material might have
a very low temperature causing issues with briquetting density and resulting in lots
of fine particles and chips.
[0012] Further, the recycling of hot fines to the briquetting machines cools the DRI significantly
due to the screening operations.
[0013] Further, inventors saw a problem in future operations with partial hydrogen usage
that the carbon formed in the reduction zone itself will decrease substantially due
to the thermodynamic conditions. This in combination with the endothermic reduction
reactions which cools the beds delimit the amount of carbon which can be added to
the product since the maximum amount of carbon which can be added via natural gas
injection in the lower furnace zone is limited by the temperature required for briquetting.
A small amount of carbon in the product is important for downstream EAF operation
to ensure a foaming slag, and therefore it is desired to have a residual carbon content
in the product, especially when using a reduction gas containing hydrogen.
[0014] Further, the inventors took into consideration that there is no way of independently
controlling the temperature of the material in the reactor and the temperature of
the material going from the reactor to the briquetting press. If the temperature of
the material arriving at the briquetting press needs to be increased, according to
the state of the art the temperature of the entire reactor must be increased. However,
this increases the risk of clustering within the furnace and decreases the energy
efficiency.
[0015] The inventors therefore found that it is advantageous to increase the temperature
of the DRI before it reaches the briquetting press thus not requiring adjustment of
the temperature in the reactor. Further, the inventors identified the need for means
of balancing the temperatures between the feed channels as well as means of heating-up
the recycled fine particles before entering the briquetting press.
[0016] In order to achieve the aforesaid and to avoid the disadvantages in the state of
the art, feed channels and/or hot fine particles are modified in such a way that induction
heating of these materials is carried out in the feed channels to control the temperature
before entering the briquetting press.
[0017] Due to this temperature control the DRI temperature is decoupled from the temperature
in the reactor allowing the reactor to be run at the optimum conditions for the direct
reduction. the temperature of the hot briquetting can be chosen independently with
the optimum temperature for this process.
[0018] The temperature of the DRI is monitored and adjusted in an induction process which
allows for controlled increase of the DRI temperature to a target temperature irrespective
of the specific feed channel. Therefore, the briquetting presses are fed with DRI
of the same temperature resulting in uniform properties of the briquettes.
[0019] It was found that by increasing the temperature of the DRI by 25°C the occurrence
of HDI chips and fines can be decreased by several percent. This leads to an increased
yield providing higher value to customers.
[0020] As a remarkable side-effect it was found that operating at a higher briquetting temperature
allows to use a higher quantity of lower grade ores which may become more important
in the future. Use of lower grade ore further saves costs.
[0021] Further, being able to reheat the DRI additional carbon can be added in the transition
zone via natural gas and/or other carbon containing gases like biogas, thus compensating
for both the loss of carbon in the reactor and cooling due to hydrogen usage.
[0022] It is preferred to use sustainable energy sources for the induction process, for
instance wind and/or solar energy and/or energy from solid oxide fuel cells. In periods
with not sufficient sustainable energy, stored hydrogen may be used for these fuel
cells and for the direct reduction of the iron ore.
[0023] In an embodiment an induction heating device is installed around each feed channel.
[0024] In an advantageous embodiment it is recommended that the system or induction heating
device for each feed channel is designed to allow for a temperature increase of up
to 100°C of the DRI material. Typically, a temperature increase of 20 °C of the DRI
material is used.
[0025] In a further advantageous embodiment the induction heating device for each system
of each feed channel is designed for a specific energy input of between 2 to 18 kWh/t
HBI, preferably between 2 to 8 kWh/t HBI during conventional operation with higher
grade ores and between 8 to 18 kWh/t HBI when operating with lower grade ores or in
a hybrid mode of operation with hydrogen.
[0026] In particular the invention pertains to a process for the manufacturing of hot briquetted
iron (HBI) from direct reduced iron (DRI) wherein iron ore is direct reduced in a
reactor by a reducing gas consisting of natural gas and/or hydrogen and/or carbon
monoxide under elevated temperatures and discharging the direct reduced iron to at
least one briquetting device where briquettes are pressed from the direct reduced
iron wherein the direct reduced iron is after leaving the reactor and before briquetting
is heated to a target briquetting temperature.
[0027] In an embodiment fines after briquetting are recovered downstream of the briquetting
press and heated to a target briquetting temperature before being reintroduced to
the briquetting press.
[0028] In an embodiment the DRI and/or the fines are heated by induction heating.
[0029] In an embodiment the DRI and the fines are heated separately or mixed together and
heated together after mixing.
[0030] In an embodiment the induction heating is positioned around a duct or channel conveying
the DRI or the fines or both to the briquetting press.
[0031] In an embodiment the DRI and/or the fines are heated to a temperature above 700°C
and preferably between 700°C and 800°C before briquetting.
[0032] In an embodiment sustainable energy sources for the induction process are used and
preferably wind energy or energy from solid oxide fuel cells is used wherein the hydrogen
used for these fuel cells is from the same source as the hydrogen used for the direct
reduction of the iron ore.
[0033] In an embodiment the induction heating device is designed for an energy input into
the material of between 2 to 18 kWh/t HBI and preferably between 2 to 8 kWh/t HBI
when treating higher grade materials and 8 to 18 kWh/t HBI when treating lower grade
ores or operating in a hybrid mode of operation with hydrogen.
[0034] So for a reduction furnace with a capacity of 2 million tonnes per year and 7 feed
legs, a nominal power input into the material of 75 - 700 kW per feed leg would be
required.
[0035] In an embodiment the induction heating device for each feed channel is designed to
allow for a temperature increase of at least 100°C or more of the DRI material wherein
preferably it is designed for a temperature increase of 20-50 °C of the DRI material
when treating high grade ores in a conventional process and between 50 -100 °C when
treating lower grade materials or operating in a hybrid mode of operation with hydrogen.
[0036] The invention is hereinafter explained by way of examples and the accompanying drawings,
wherein the drawing show:
- Figure 1:
- The feed channel temperature in degree Fahrenheit in relation to density of the briquettes;
- Figure 2:
- the relative strength of the briquettes in relation to the temperature;
- Figure 3:
- the decreasing temperature of the material with higher carbon content;
- Figure 4:
- a schematic view on a modified feed channel;
- Figure 5:
- the percentage of chips and fines of HBI in relation to the feed channel temperature;
- Figure 6:
- the pellet grade limitation with current feed channel temperatures in comparison to
the pellet grade limitation with higher feed channel temperatures;
- Figure 6a:
- the briquette density in relation to the Fe content in pellet before reduction at
700°C;
- Figure 6b:
- the briquette density in relation to the Fe content in pellet before reduction at
800°C;
- Figure 7:
- the comparison of the prior art with 100 % natural gas in comparison to hydrogen natural
gas operation with reduced carbon content and with a desired carbon content in hydrogen/natural
gas operation according to the invention;
- Figure 8:
- possible installation positions, for briquetting plants, where fines resulting from
the briquetting process are fed back to the DRI.
[0037] In Figure 1 the feed channel temperature in degrees Fahrenheit and the percentage
of briquettes with a density above 5 t/m
3 is shown. From the chart it can be derived, that the higher the feed channel temperature
is, the higher the density of the briquettes is, wherein the increase in density is
not linear but can be much decreased by a temperature rise of 30° F. Further, in Figure
2 the relative strength of the briquettes is shown in relation to the feed channel
temperature, and it shows that higher temperatures lead to a higher relative strength.
[0038] In a process in which natural gas is introduced leading to partial hydrogen usage,
the amount of carbon formed in the reduction zone itself will decrease substantially
due to the thermodynamic conditions. This combined with the endothermic reduction
reaction which cools the bed will limit the amount of carbon which can be added to
the product since the maximum amount of carbon which can be added via natural gas
injected into the lower furnace zone will be limited by the temperature required for
briquetting.
[0039] In Figure 3 this can be clearly seen as the higher the carbon content the lower is
the material temperature in the feed channel, which is lower than the values shown
in Figures 1 and 2 for a sufficient density and sufficient relative strength of the
briquettes.
[0040] The invention aims to solve the problem of the different temperatures originating
from on one hand the DRI reactor being operated in the best mode, with a higher hydrogen
content in the reducing gas and introducing carbon by injection of natural gas and
on the other hand operating the briquetting so that briquettes can be formed with
sufficient strength and density.
[0041] According to Figure 4 this is achieved by induction heating of the DRI or sponge
iron in the feed channel to the briquetting machine so that the sponge iron or DRI
can be adjusted to an ideal temperature which normally lies above 1300° Fahrenheit.
[0042] As it can be seen in Figure 5 the amount of chips and fines in the production of
the briquettes can be decreased considerably by a temperature increase of only 25°C.
This is due to the fact, that by increasing the temperature, the density as well as
the strength of the briquettes can be enhanced.
[0043] In Figure 6 the pellet grade limitation with the current feed channel temperatures
is shown in comparison to the higher feed channel temperatures according to the invention.
As a further advantage it was found that an iron ore with an Fe-content in the ore
pellets before the reduction of about 66.5 % lead to the required briquette density
of 5 tons per m
3. This is achieved normally at a briquette temperature of 700°C (1292° F) (Fig. 6a).
It was found that by rising the briquetting temperature by 100°C to 800°C (1472 F)
(Fig. 6b), the same density can be achieved with an iron ore with a remarkable lower
Fe-content allowing the use of a higher quantity of lower grade ores which can be
important in the future and provides cost advantages.
[0044] In Figure 7 three different operating systems for operating the reactor are shown.
On the left side the current situation is shown wherein 100 % natural gas is used
for reforming to produce a syn gas for reduction into the reactor. As it can be seen
the product has a carbon content of about 1.5 weight-% and the operating temperature
of the DRI is about 670 - 700°C. In the middle a hydrogen/natural gas mixed operation
is shown with a maximum carbon content while keeping the DRI temperature at a minimum
of about 670°C. In this case the product has a carbon content below 1 wt-%.
[0045] On the right a hydrogen/natural gas mixed operation is shown. The operating conditions
are shown in the middle but the carbon content should be kept at about 1.5 wt.%, which
is required for the product. The operating conditions shown in the middle picture
do not achieve a high enough carbon content. Achieving a carbon content of about 1.2
- 1.5 wt-% is possible at a temperature of the DRI of 600°C which is too cold for
briquetting.
[0046] According to the present invention, the operation route which is shown on the right
can be chosen, so that a product with a sufficient carbon content is obtained. The
lower output temperature from the reactor is compensated according to the invention
by induction heating of the DRI before briquetting. In a preferred embodiment the
induction heating of the DRI is performed in the feed channel from the reactor to
the briquetting machine. It is advantageous to design the induction heating system
to allow for a temperature increase of at least 100°C or more of the DRI material.
It can be sufficient to allow for an increase in temperature of up to 100°C of the
DRI material. It is expected that during typical operation the increase of the material
temperature by 20°C is sufficient.
[0047] For the purpose of sufficient heating of DRI the temperature of the DRI can be monitored
before reaching the induction heating zone and if required heated up to a predetermined
temperature. This allows that all materials of all feed channels arrive at the briquetting
machines at the predetermined temperature.
[0048] Figure 8 shows different installation possibilities of induction heating shown for
briquetting plants where fines are fed back to the briquetting machine. Due to the
differences in temperature between the DRI (Temperature T1) and the fines (Temperature
T2), the resulting DRI/fines mixture temperature (Temperature T3) depends on the relative
amounts of fine recycled and the temperature of both the incoming DRI and the fines.
In the first example, only the temperature of the DRI is adjusted (2) to control the
temperature of the mixture of DRI and fines which is fed into the briquetting press.
In the second example, an additional heating source (5) is installed on the fines
recycling line in order to allow for the control of the temperature of the DRI and
the fines independently.
[0049] Induction heating is the preferred heating method as induction heating has a high
degree of efficiency. In addition, a high mass flow of DRI or sponge iron material
can be treated very effectively. The electric power needed for that induction heating
can be derived from sustainable energy sources so that the overall process is already
more environmentally friendly than conventional steel making.
[0050] For example, for the production of the electric energy for the induction heating
the same hydrogen source can be used as for the reduction or electrical energy form
wind energy or the like can be used. Further, feed channel pipes can be easily adapted
to an induction heating process by arranging induction coils on the pipes which is
much easier achieved and safer than for example heating by combustible energy sources.
[0051] The expected energy input into the material for the required temperature increase
may be between 2-18 kWh/t HBI, with an expected range of 2 - 8 kWh/t HBI when treating
higher grade materials and a range of 8 to 18 kWh/t HBI when treating lower grade
ores or operating in a mixed hydrogen/natural gas operation mode.
1. Process for the manufacturing of hot briquetted iron (HBI) from direct reduced iron
(DRI) wherein iron ore is direct reduced in a reactor by a reducing gas consisting
of natural gas and/or hydrogen and/or carbon monoxide under elevated temperatures
and discharging the direct reduced iron to at least one briquetting device where briquettes
are pressed from the direct reduced iron
characterized in that
the direct reduced iron is after leaving the reactor and before briquetting is heated
to a target briquetting temperature.
2. Process according to claim 1, characterized in that fines after briquetting are recovered downstream of the briquetting press and heated
to a target briquetting temperature before being reintroduced to the briquetting press.
3. Process according to claim 1 or 2, characterized in that the DRI and/or the fines are heated by induction heating.
4. Process according to one of the preceding claims, characterized in that the DRI and the fines are heated separately or mixed together and heated together
after mixing.
5. Process according to one of the preceding claims claim, characterized in that the induction heating is positioned around a duct or channel conveying the DRI or
the fines or both to the briquetting press.
6. Process according to one of the preceding claims, characterized in that the DRI is heated to a temperature above 700°C and preferably between 700 and 750
C when processing higher grade ores during conventional operation and between 750
and 800 C when using lower grade raw materials or operating in a hybrid mode of operation
with hydrogen.
7. Process according to one of the preceding claims, characterized in that sustainable energy sources for the induction process are used and preferably wind
and/or solar energy and/or energy from solid oxide fuel cells is used wherein the
hydrogen used for these fuel cells is from the same source as the hydrogen used for
the direct reduction of the iron ore.
8. Process according to one of the preceding claims, characterized in that the induction heating device is designed for an energy input into the DRI of about
2 - 18 kWh/t HBI.
9. Process according to one of the preceding claims, characterized in that the induction heating device for each feed channel is designed to allow for a temperature
increase of at least 100°C or more of the DRI material wherein preferably it is designed
for a temperature increase of 20 °C of the DRI material when treating higher grade
materials and temperature increase of 50 to 100°C when treating lower grade ores or
operating in a hybrid mode of operation with hydrogen.